In the discussion which follows, the invention will be described with reference to the encoding and decoding of the audio frequency portion of a television broadcast signal, but it is to be clearly understood that the scope of the invention is not limited to this application.
The encoding and decoding of broadcast signals to enforce the collection of license fees from subscribers to the broadcast signals is well known.
One such system suitable for encoding television broadcast signals is described in applicants' above-mentioned co-pending South African patent application No. 93/2436. In this system, the video portion of the television broadcast signal is encoded, while the audio portion of the broadcast signal remains "in clear". This is disadvantageous as pirate viewers are still able to hear the audio frequency portion of the broadcast signal.
As far as the applicant is aware, all known systems for the encoding and decoding of audio transmission signals require the audio signals to be mixed down to baseband frequencies prior to being encoded and decoded.